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Version 1.4

General Certificate of Education (A-level) January 2013

Chemistry

(Specification 2420)

CHEM4

Unit 4: Kinetics, Equilibria and Organic Chemistry

Final

Mark Scheme

Report on the Examination Mark schemes are prepared by the Principal Examiner and considered, together with the relevant questions, by a panel of subject teachers. This mark scheme includes any amendments made at the standardisation events which all examiners participate in and is the scheme which was used by them in this examination. The standardisation process ensures that the mark scheme covers the students’ responses to questions and that every examiner understands and applies it in the same correct way. As preparation for standardisation each examiner analyses a number of students’ scripts: alternative answers not already covered by the mark scheme are discussed and legislated for. If, after the standardisation process, examiners encounter unusual answers which have not been raised they are required to refer these to the Principal Examiner. It must be stressed that a mark scheme is a working document, in many cases further developed and expanded on the basis of students’ reactions to a particular paper. Assumptions about future mark schemes on the basis of one year’s document should be avoided; whilst the guiding principles of assessment remain constant, details will change, depending on the content of a particular examination paper.

Further copies of this Mark Scheme are available from: aqa.org.uk Copyright © 2013 AQA and its licensors. All rights reserved. Copyright AQA retains the copyright on all its publications. However, registered centres for AQA are permitted to copy material from this booklet for their own internal use, with the following important exception: AQA cannot give permission to centres to photocopy any material that is acknowledged to a third party even for internal use within the centre. Set and published by the Assessment and Qualifications Alliance. The Assessment and Qualifications Alliance (AQA) is a company limited by guarantee registered in England and Wales (company number 3644723) and a registered charity (registered charity number 1073334). Registered address: AQA, Devas Street, Manchester M15 6EX.

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Mark Scheme – General Certificate of Education (A-level) Chemistry – Unit 4: Kinetics, Equilibria and Organic Chemistry – January 2013

3

Question Marking Guidance Mark Comments

1(a)(i) k =

222

5

106.2)102.4(104.8

−−

−

×××

× OR 23

5

106.2)1076.1(104.8

−−

−

×××

×

= 1.8(3)

mol–2 dm+6 s–1

1

1

1

Mark is for insertion of numbers into a correctly rearranged rate equ , k = etc. If upside down, score only units mark from their k AE (-1) for copying numbers wrongly or swapping two numbers

Any order If k calculation wrong, allow units consequential to their k = expression

1(a)(ii) 5.67 × 10–4 (mol dm–3s–1) OR their k × 3.1 × 10-4 1 Allow 5.57 × 10–4 to 5.7 × 10–4

1(b)(i) 2 or second or [D]2 1

1(b)(ii) 0 or zero or [E]0 1

1(c)(i) Step 1 or equation as shown 1 Penalise Step 2 but mark on

1(c)(ii)

H3C C

CH3

Br

CH3 or (CH3)3C Br

Ignore correct partial charges, penalise full / incorrect partial charges

1 If Step 2 given above, can score the mark here for

(CH3)3C OH allow :OH– (must show lp)

If SN2 mechanism shown then no mark (penalise involvement of :OH– in step 1)

Ignore anything after correct step 1


4


2(a)(i) [H+][OH–] OR [H3O+][OH–] Ignore (aq) 1 Must have [ ] not ( )

2(a)(ii) √3.46 × 10–14 (= 1.86 × 10–7) pH = 6.73

1 1

If no square root, CE=0

Must be 2dp

2(a)(iii) [H+] = 10-11.36 (= 4.365 × 10–12 OR 4.37 × 10–12) Kw = [4.365 ×10–12 OR 4.37 ×10–12 × 0.047] = 2.05 × 10–13

Allow 2.05 × 10–13 - 2.1 × 10–13

1 1

Mark for working Mark for answer

Ignore units

2(b)(i) HCOOH HCOO– + H+

OR HCOOH + H2O HCOO– + H3O+

1 Must have but ignore brackets.

Allow HCO2- or CHOO- ie minus must be on oxygen, so

penalise COOH-

2(b)(ii) Ka =

]HCOOH[]HCOO][[H −+

OR ]HCOOH[

]HCOO][O[H3−+

1 Must have all brackets but allow ( )

Must be HCOOH etc.

Allow ecf in formulae from 2(b)(i)

2(b)(iii) M1

M2

M3

Ka = ]HCOOH[

][H 2+

([H+]2 = 1.78 × 10–4 × 0.056 = 9.97 × 10–6)

[H+] = 3.16 × 10–3

pH = 2.50 allow more than 2 dp but not fewer

1

1

1

Allow HA or HX etc. Allow [H+] = √(Ka × [HA]) for M1

Mark for answer

Allow correct pH from their wrong [H+] here only If square root shown but not taken, pH = 5.00 can score max 2 for M1 and M3


5

2(b)(iv) M1 M2 M3

Decrease Mark M1 independently Equm shifts/moves to RHS OR more H+ OR Ka increases OR more dissociation To reduce temperature or oppose increase/change in temperature

1 1

1

Only award M3 following correct M2

2(c)(i) M1

M2

M3

[H+] = ][X[HX] x Ka

- OR pH = pKa – log

][X[HX]

-

2-

-2-4

10 1.8410 2.35 x 10 1.78

××× OR pH = 3.75 – log

2-

-2

10 1.8410 2.35

×

×

(= 2.27 × 10–4)


1

1

1

If [HX]/[X-] upside down, no marks

pH calc NOT allowed from their wrong [H+] here

2(c)(ii) M1 M2

M3

M4

Mol H+ added = 5.00 × 10–4

Mol HCOOH = 2.40 × 10–2 and Mol HCOO– = 1.79 × 10–2

[H+] (=][X[HX] x Ka

-) =

2-

-2-4

10 1.7910 2.40 x 10 1.78

××× ( = 2.39 × 10–4)

OR pH = 3.75 – log2-

-2

10 1.7910 2.40

×

×


1 1 1

1

Mark on from AE in moles of HCl (eg 5 x 10-3 gives pH = 3.42 scores 3)

If either wrong no further marks except AE (-1) OR if ECF in mol acid and/or mol salt from (c)(i), can score all 4

If [HX]/[X-] upside down here after correct expression in (c)(i), no further marks If [HX]/[X-] upside down here and is repeat error from (c)(i), max 3 (pH = 3.88 after 3.86 in 2(c)(i)) pH calc NOT allowed from their wrong [H+] here


6


3(a)(i) 3CH3OH HOCH2CH(OH)CH2OH

1 1

Not molecular formula

3(a)(ii) → 19CO2 + 19H2O C17H35COOCH3 + 27½ or 55/2 O2

1 1

Or doubled

Consequential on correct right-hand side

3(b)(i) A 0.7 Ethanol 6.4 Water 3.6

1 1 1

3(b)(ii) No effect Equal moles on each side of equation OR V cancels

1 1

If wrong, CE= 0

Ignore moles of gas

3(b)(iii) M1 Kc = 2

52

22

]OHHC][A[]OH][DEM[

1 Must have all brackets but allow ( )

3(b)(iv) M2 M3 M4

2

2

).27( 0.85(3.4) x 1.2×

0.55 (min 2dp) No units

1

1

1

If Kc wrong can only score M4 for units consequential to their Kc working in (b)(iv)


7


4(a)(i)

1

These four only

4(a)(ii)

H3N C

CH3

H

COO

1

Allow –NH3+ and +NH3–

4(a)(iii) 2-amino-3-hydroxybutanoic acid

Do not penalise commas or missing hyphens 1 Ignore 1 in butan-1-oic acid

Penalise other numbers

4(a)(iv)

H3N C

(CH2)4

H

COOH

NH3

1

Allow –NH3+ and +NH3–

H2N C

CH3

H

C

O

N

H

C

CHOH

H

C

CH3

O

N

H

C

(CH2)4

H

COOH

NH2


8

4(b)(i) Condensation 1 Allow polyester

4(b)(ii) propane-1,3-diol 1 Must have e

Allow 1,3-propanediol

4(c)(i) Addition 1 Not additional

4(c)(ii) C

H

H

C

F

F

and

C

F

F

C

CF3

F OR

C

F

F

C

F

F

and

C

F3C

F

C

H

H

1 for each

structure within

each pair

Allow monomers drawn either way round

Allow bond to F in CF3

4(d) c C-C or C-F bonds too strong

1

1

If wrong, CE = 0


9


5(a)(i) Single/one (intense) peak/signal OR all H or all C in same environment OR 12 equiv H or 4 equiv C OR Upfield / to the right of (all) other peaks OR well away from others OR doesn’t interfere with other peaks OR Low bp OR volatile OR can easily be removed

2 Do not allow non-toxic or inert (both given in Q) Any 2 from three Ignore peak at zero

Ignore cheap

Ignore non-polar

Ignore mention of solubility

5(a)(ii) Si

CH3

H3C CH3

CH3

1 Allow Si(CH3)4

5(b)(i) C

O

CH3

or with sticks or C

O

CR

H

1 Ignore any group joined on other side of CO Ignore missing trailing bond Ignore charges

5(b)(ii) CH3 CH2 O or with sticks

1 Ignore any group joined on other side of –O– Ignore missing trailing bond Ignore charges as if MS fragment

5(b)(iii) CH2 CH2 CO

O

or with sticks

1 Ignore missing trailing bond Ignore charges as if MS fragment

5(b)(iv) CH3 CH2 O CH2 CH2 C

O

CH3

1


10

5(c)(i) Check structure has 6 carbons

H3C C

CH3

CH3

C

O

O CH3

O C

CH3

CH3

CH3C

O

H3C

1

1

Allow (CH3)3CCOOCH3 or (CH3)3CCO2CH3

Allow CH3COOC(CH3)3 or CH3CO2C(CH3)3

5(c)(ii) Check structure has 6 carbons

C C

CH3

H

C

O

OHH3C

H

CH3

1

Allow (CH3)2CHCH(CH3)COOH or (CH3)2CHCH(CH3)CO2H Penalise C3H7

5(c)(iii) Check structure has 6 carbons

OHHO

OR

O

H2CH2C CH2

O

CH2

CH2CH2

1

Allow H3C

CCH3

O OC

H3C CH3

OR

O

C C

O

H3C

CH3 CH3

CH3


11


6

In each section • If wrong or no reagent given, no marks for any observations;

• Penalise incomplete reagent or incorrect formula – but mark observations

• Mark each observation independently

• Allow no reaction for no change / no observable reaction in all three parts, but not none or nothing

• Q says one test. If two tests are given, score zero

6(a) K2Cr2O7 / H+ KMnO4 / H+ Lucas test (ZnCl2 / HCl)

1 Allow acidified potassium manganate and acidified potassium dichromate without oxidation numbers R

Primary alcohol

(Orange) goes green

Penalise wrong starting colour

(purple) goes colourless /

decolourises allow goes brown

No cloudiness 1

S Tertiary alcohol

no change / no observable

reaction


reaction Rapid cloudiness 1

6(b) Na2CO3 / NaHCO3 named carbonate metal eg Mg named indicator 1 PCl5 PCl3

SOCl2 Named alcohol + HCl / H2SO4

T ester


reaction


reaction no effect 1


reaction

no change / no observable reaction

U Acid

Effervescence or (CO2) gas formed

Effervescence or (H2) gas formed acid colour 1 Fumes / (HCl)

gas formed Sweet smell


12


In each section • If wrong or no reagent given, no marks for any observations;

• Penalise incomplete reagent or incorrect formula – but mark observations

• Mark each observation independently

• Allow no reaction for no change / no observable reaction in all three parts, but not none or nothing

• Q says one test. If two tests are given, score zero

6(c) Fehling’s / Benedict’s

Tollens’ / [Ag(NH3)2]+ K2Cr2O7 / H+ 1 I2 / NaOH

V Ketone


reaction


reaction


reaction 1

Yellow ppt

W aldehyde Red ppt Silver mirror

(Orange) goes green

Penalise wrong starting colour

1 no change / no observable reaction


13


7(a) Sn / HCl OR Fe / HCl not conc H2SO4 nor any HNO3

Ignore subsequent use of NaOH

Equation must use molecular formulae C6H4N2O4 + 12 [H]

→ C6H8N2 + 4H2O

C C

O

N

H

N

O H

1

1 1

2

Ignore reference to Sn as a catalyst with the acid

Allow H2 (Ni / Pt) but penalise wrong metal But NOT NaBH4 LiAlH4 Na / C2H5OH

12[H] and 4H2O without correct molecular formula scores 1 out of 2 Allow ….. + 6H2 if H2 / Ni used

Allow –CONH― or ―COHN― or ―C6H4― Mark two halves separately : lose 1 each for

• error in diamine part • error in diacid part • error in peptide link • missing trailing bonds at one or both ends • either or both of H or OH on ends

Ignore n

7(b) H2 (Ni / Pt) but penalise wrong metal CH2

In benzene 120o In cyclohexane 109o 28’ or 109½o If only one angle stated without correct qualification, no mark awarded

1 1 1 1

NOT Sn / HCl, NaBH4 etc.

Allow 108o - 110o


14

7(c)(i) Nucleophilic addition

CH3CH2 CO

CH3

CH3CH2 C

O

CH3

H

CH3CH2 C

M1

M2

M3H

H

M4 for lp, arrow and H+

OH

H

CH3

1

4

• M2 not allowed independent of M1, but allow M1 for correct attack on C+

• + rather than δ+ on C=O loses M2 • M3 is for correct structure including minus sign but

lone pair is part of M4 • Allow C2H5

• M1 and M4 include lp and curly arrow • Allow M4 arrow to H in H2O (ignore further arrows)

7(c)(ii) M1 Planar C=O (bond/group)

M2 Attack (equally likely) from either side M3 (about product): Racemic mixture formed OR 50:50 mixture

or each enantiomer equally likely

1

1

1

Not just planar molecule

Not just planar bond without reference to carbonyl


15


8(a)(i) CH3COCl + C6H6 → C6H5COCH3 + HCl

OR 1 Not molecular formulae Not allow C6H5CH3CO

Ignore number 1 in name but penalise other numbers

Allow RHS as

CH3 C

O

Cl AlCl3δ+ δ

Allow + on C or O in equation but + must be on C in mechanism below Ignore curly arrows in balanced equation even if wrong

CH3COCl +

→ C

O

CH3

+ HCl

phenylethanone AlCl3 can be scored in equation

CH3COCl + AlCl3 CH3CO + AlCl4

1

1

1


16

8(a)(ii) Electrophilic substitution

C

O

CH3

H

COCH3

M1

M2

M3

OR

CH

COCH3

M1

M2

M3

+

O

CH3

1

3

• M1 arrow from within hexagon to C or to + on C • + must be on C of CH3CO in mechanism • + in intermediate not too close to C1 • Gap in horseshoe must be centred approximately

around C1 • M3 arrow into hexagon unless Kekule • Allow M3 arrow independent of M2 structure,

ie + on H in intermediate loses M2 not M3 • Ignore base removing H for M3

8(b) Electron pair donor or lone pair donor

H3C C

O

O C

O

CH3 (acid) anhydride

1

1

1

Allow donator

Allow lone pair used in description of (dative) bond formation

Allow (CH3CO)2O

Allow ethanoic anhydride but not any other anhydride


17

General principles applied to marking CHEM4 papers by CMI+ (January 2013)

It is important to note that the guidance given here is generic and specific variations may be made at individual standardising meetings in the context of particular questions and papers. Basic principles

• Examiners should note that throughout the mark scheme, items that are underlined are required information to gain credit. • Occasionally an answer involves incorrect chemistry and the mark scheme records CE = 0, which means a chemical error has occurred

and no credit is given for that section of the clip or for the whole clip. •

A. The “List principle” and the use of “ignore” in the mark scheme

If a question requires one answer and a student gives two answers, no mark is scored if one answer is correct and one answer is incorrect. There is no penalty if both answers are correct. N.B. Certain answers are designated in the mark scheme as those which the examiner should “Ignore”. These answers are not counted as part of the list and should be ignored and will not be penalised.

B. Incorrect case for element symbol

The use of an incorrect case for the symbol of an element should be penalised once only within a clip. For example, penalise the use of “h” for hydrogen, “CL” for chlorine or “br” for bromine.

C. Spelling

In general • The names of chemical compounds and functional groups must be spelled correctly to gain credit. • Phonetic spelling may be acceptable for some chemical terminology.

N.B. Some terms may be required to be spelled correctly or an idea needs to be articulated with clarity, as part of the “Quality of Language” (QoL) marking. These will be identified in the mark scheme and marks are awarded only if the QoL criterion is satisfied.


18

D. Equations

In general • Equations must be balanced. • When an equation is worth two marks, one of the marks in the mark scheme will be allocated to one or more of the reactants or products. This

is independent of the equation balancing. • State symbols are generally ignored, unless specifically required in the mark scheme.

E. Reagents

The command word “Identify”, allows the student to choose to use either the name or the formula of a reagent in their answer. In some circumstances, the list principle may apply when both the name and the formula are used. Specific details will be given in mark schemes. The guiding principle is that a reagent is a chemical which can be taken out of a bottle or container. Failure to identify complete reagents will be penalised, but follow-on marks (e.g. for a subsequent equation or observation) can be scored from an incorrect attempt (possibly an incomplete reagent) at the correct reagent. Specific details will be given in mark schemes. For example, no credit would be given for

• the cyanide ion or CN– when the reagent should be potassium cyanide or KCN; • the hydroxide ion or OH– when the reagent should be sodium hydroxide or NaOH; • the Ag(NH3)2

+ ion when the reagent should be Tollens’ reagent (or ammoniacal silver nitrate). In this example, no credit is given for the ion, but credit could be given for a correct observation following on from the use of the ion. Specific details will be given in mark schemes.

In the event that a student provides, for example, both KCN and cyanide ion, it would be usual to ignore the reference to the cyanide ion (because this is not contradictory) and credit the KCN. Specific details will be given in mark schemes.

F. Oxidation states

In general, the sign for an oxidation state will be assumed to be positive unless specifically shown to be negative.


19

G. Marking calculations

In general • A correct answer alone will score full marks unless the necessity to show working is specifically required in the question. • An arithmetic error may result in a one mark penalty if further working is correct. • A chemical error will usually result in a two mark penalty.

H. Organic reaction mechanisms

Curly arrows should originate either from a lone pair of electrons or from a bond. The following representations should not gain credit and will be penalised each time within a clip.

CH3 Br CH3 Br CH3 Br.. . .

OH OH.. _ _

:

For example, the following would score zero marks

H3C C

H

H

Br

HO

When the curly arrow is showing the formation of a bond to an atom, the arrow can go directly to the relevant atom, alongside the relevant atom or more than half-way towards the relevant atom.

In free-radical substitution

• The absence of a radical dot should be penalised once only within a clip. • The use of double-headed arrows or the incorrect use of half-headed arrows in free-radical mechanisms should be penalised once only within

a clip In mass spectrometry fragmentation equations, the absence of a radical dot on the molecular ion and on the free-radical fragment would be considered to be two independent errors and both would be penalised if they occurred within the same clip.


20

I. Organic structures

In general • Displayed formulae must show all of the bonds and all of the atoms in the molecule, but need not show correct bond angles. • Bonds should be drawn correctly between the relevant atoms. This principle applies in all cases where the attached functional group contains

a carbon atom, e.g nitrile, carboxylic acid, aldehyde and acid chloride. The carbon-carbon bond should be clearly shown. Wrongly bonded atoms will be penalised on every occasion. (see the examples below)

• The same principle should also be applied to the structure of alcohols. For example, if students show the alcohol functional group as C ─ HO, they should be penalised on every occasion.

• Latitude should be given to the representation of C ─ C bonds in alkyl groups, given that CH3─ is considered to be interchangeable with H3C─ even though the latter would be preferred.

• Similar latitude should be given to the representation of amines where NH2─ C will be allowed, although H2N─ C would be preferred. • Poor presentation of vertical C ─ CH3 bonds or vertical C ─ NH2 bonds should not be penalised. For other functional groups, such as ─ OH

and ─ CN, the limit of tolerance is the half-way position between the vertical bond and the relevant atoms in the attached group. By way of illustration, the following would apply.

CH3 C

C

CH3

C

CH3CH2

OH C

C

OH

allowed allowed not allowed not allowed not allowed

NH2 C

C

NH2

NH2

NH2

NO2

allowed allowed allowed allowed not allowed


21

CN C

C

CN

COOH C

C

COOH

C

COOH

not allowed not allowed not allowed not allowed not allowed

CHO C

C

CHO

C

CHO

COCl C

C

COCl

C

COCl

not allowed not allowed not allowed not allowed not allowed not allowed

• In most cases, the use of “sticks” to represent C ─ H bonds in a structure should not be penalised. The exceptions will include structures in

mechanisms when the C ─ H bond is essential (e.g. elimination reactions in haloalkanes) and when a displayed formula is required. • Some examples are given here of structures for specific compounds that should not gain credit

CH3COH for ethanal CH3CH2HO for ethanol OHCH2CH3 for ethanol C2H6O for ethanol CH2CH2 for ethene CH2.CH2 for ethene CH2:CH2 for ethane

N.B. Exceptions may be made in the context of balancing equations • Each of the following should gain credit as alternatives to correct representations of the structures.

CH2 = CH2 for ethene, H2C=CH2 CH3CHOHCH3 for propan-2-ol, CH3CH(OH)CH3


22

J. Organic names

As a general principle, non-IUPAC names or incorrect spelling or incomplete names should not gain credit. Some illustrations are given here.

but-2-ol should be butan-2-ol 2-hydroxybutane should be butan-2-ol butane-2-ol should be butan-2-ol 2-butanol should be butan-2-ol 2-methpropan-2-ol should be 2-methylpropan-2-ol 2-methylbutan-3-ol should be 3-methylbutan-2-ol 3-methylpentan should be 3-methylpentane 3-mythylpentane should be 3-methylpentane 3-methypentane should be 3-methylpentane propanitrile should be propanenitrile aminethane should be ethylamine (although aminoethane can gain credit) 2-methyl-3-bromobutane should be 2-bromo-3-methylbutane 3-bromo-2-methylbutane should be 2-bromo-3-methylbutane 3-methyl-2-bromobutane should be 2-bromo-3-methylbutane 2-methylbut-3-ene should be 3-methylbut-1-ene difluorodichloromethane should be dichlorodifluoromethane

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